The present invention relates to a method and apparatus for acoustic logging; and more particularly, to a method and apparatus using propagating ultrasonic energy to detect the presence of fractures and anomalies in an earth formation surrounding a borehole.
Surface-breaking fractures and other anomalies occurring at the wall of a borehole traversing an earth formation are of interest since many reservoirs produce hydrocarbons through fractured formations. Vertical fractures are of greatest interest to the industry, since horizontal fractures tend to be closed by overburden and hence do not facilitate production. The wall of a borehole is a remote and particularly hostile environment in which to detect fractures, however, and the success of known devices and techniques has been variable.
One known method is based on an amplitude reduction of acoustic waves traveling longitudinally between the transmitter and receiver of an axially oriented array. Compressional or shear waves cannot effectively be used in this method. Theoretically, boundary waves--those waves traveling along the borehole wall with characteristics that depend on the interaction of the wall and borehole fluid--are responsive to vertical fractures. In practice, however, boundary waves do not respond to fractures under certain conditions of interest. See Koerperich, Investigation of Acoustic Boundary Waves and Interference Patterns as Techniques for Detecting Fractures, 30 J. Pet. Tech. 1199 (1978).
Another known method is based on interference patterns of reflected or mode converted energy that originate from acoustic discontinuities such as fractures or bed boundaries, and travel longitudinally in the borehole. As the tool approaches and moves away from the discontinuity, the pattern becomes visible as a "w" or "v" shape on specific types of displays if a mode conversion occurs. See U.K. Pat. No. 1,116,406 (Shell International Research, Matschappij N. V., published June 6, 1968). Interference patterns generally do not originate at fractures which are nearly vertical, however. See Koerperich, supra.
A method for detecting vertical fractures which has received great attention is the use of an energy reduction technique based on circumferentially traveling shear waves which in vertically fractured zones normally intersect the fracture plane. An early technique is disclosed in U.S. Pat. No. 2,943,694 (Goodman, July 5, 1960). The Goodman apparatus engages the wall of the borehole, and acoustic energy is applied to a first wall location. The acoustical coupling is achieved by direct contact. Acoustic energy propagates to a second wall location where it is received by a transducer that is insensitive to longitudinal wave energy; hence, the signal which is detected at the second location is representative substantially only of shear wave energy. Cracks or fissures in the earth formations that are interposed in the shear wave transmission path reduce the amplitude of the signal obtained at the receiving transducer, relative to the signal amplitude obtained with an intervening solid formation. According to Goodman, discrimination against compressional waves may be improved by arranging the spacing and operating frequency so that the compressional and shear waves are displaced and phased by 90.degree. and by employing phase selection thereto.
A variation of the Goodman approach involves projecting narrow-beam acoustic energy into the formation at such an angle of incidence that shear wave energy is maximized while compressional wave energy is minimized. See U.S. Pat. No. 3,406,776 (Henry, Oct. 22, 1968); U.S. Pat. No. 3,585,580 (Vogel, June 15, 1971); U.S. Pat. No. 3,949,352 (Vogel, Apr. 6, 1976). The narrow beam approaches were not altogether successful. Because of variations in the shear speeds of formations of interest, the critical angle of incidence for the shear wave is variable and energy could be coupled into other wave types. Moreover, the energy reduction technique is unreliable. Acoustic energy employed in narrow-beam systems should be well above 100 kHz, i.e. 500 kHz or higher, in order to form a narrow-beam with a transducer of realistic size. At these frequencies scattering from even slight surface roughness will give large attenuation of the signal and mimic a fracture response. As a result, such a narrow-beam tool would be unduly sensitive to even slight changes in borehole conditions.
U.S. Pat. No. 3,794,976 (Mickler, Feb. 26, 1974) discloses a system and method for separating the compressional arrivals and the shear arrivals based on the use of an extremely short burst of acoustic energy (i.e. highly-dampened acoustic energy). The Mickler approach was not entirely successful in practice, possibly due to weaknesses in the energy reduction technique on which it relied.
Techniques for determining formation characteristics from both compressional and shear wave energy have been developed. For example, U.S. Pat. No. 3,775,739 (Vogel, Nov. 27, 1973) discloses the use of separate transducer sets, one oriented to produce substantially compressional waves in the formation and the other oriented to produce substantially shear waves in the formation. The presence of a vertical fracture is indicated by a reduction in the amplitude of the received shear wave without a corresponding reduction in the amplitude of the compressional wave. Acoustic energy is propagated in overlapping intervals so that the entire circumference of the borehole is surveyed.
A technique for determining formation characteristics from shear and other energy types is disclosed in U.S. Pat. No. 4,293,934 (Herolz et al, Oct. 6, 1981). See also U.S. Pat. No. 4,130,816 (Vogel et al. Dec. 19, 1978). The transmitters of the apparatus radiate compressional wave acoustic energy of an omnidirectional character in the azimuthal direction into the well fluid filling the borehole. Appreciable energy is radiated in the direction of the various critical angles of refraction. According to Herolz, the first arrival is a small amplitude compressional rock wave which contributes negligibly to the amplitude of the received signal. The next arriving wave is a refracted formation shear wave. The third wave to arrive is a compressional wave that travels directly through the borehole fluid to the receiver (a direct fluid wave). The fourth wave type is a compressional "guided fluid" wave. Transmission of the compressional "guided fluid" wave and the shear wave are impaired by an open fracture, although the guided fluid wave exhibits a high degree of attenuation when traveling along the wall past a fluid filled fracture only if the fracture is of sufficient width to cause leakage of the wave pressure into the open fracture. While the shear wave may be separately recorded and processed to the complete exclusion of later arriving waves, the guided fluid wave received after the shear wave may be separately recorded and processed with the shear.
While a number of studies indicate that a very careful analysis of sonic logs produced by such prior art techniques can be useful in locating fractures (see, e.g., Koerperich, Evaluation of the Circumferential Microsonic Log-A Fracture Detection Device, SPWLA Sixteenth Annual Logging Symposium, June 4-7 (1975); Suau & Gartner, Fracture Detection from Well Logs, 21(2) The Log Analyst 3 (1980)), at least one other study indicates that difficulty in interpreting these logs can be expected under certain circumstances (see, e.g., Hirsch, Cisar, Glass & Romanowski, Recent Experience with Wireline Fracture Detection Logs, SPE Paper 10333 (1981)).